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Tuesday. You would not believe the struggle I had and the expense to drill one 29mm hole. It must have cost me £40 and 40 miles in time and petrol by the time I had what I needed from visiting three different DIY outlets miles apart.
Finally I had a Bosch 29mm hole saw and mandrel. So I could start building a prototype out of scrap 9mm birch plywood. It took a couple of attempts to avoid the rack and the rotation collar with the focusing motor.
Then the lower section of the [temporary plywood] clamp needed to be sanded off at a very acute angle. To avoid being tipped over by the curve of the collar.
Drilling the parts on edge to allow through screws for clamping the two parts together added to the fun. I am still not completely happy with the arrangement because I have no captive nuts for the lower half. Which is why it looks a bit skewed in the images.
I stretched a rubber band over the motor shaft and fine focus wheel to simulate the toothed [timing] belt. Believe it, or not, this flimsy rubber band was able to rotate the fine focus knob with the focuser horizontal.
The lower curve on the plywood could be deepened and shaped for a sleeker look. Not to mention even better access to the brake screw and focus knobs for manual adjustment. The 29mm clamping hole must obviously be avoided but plastics and Tufnol are much stronger and stiffer materials, in smaller cross sections, than 9mm plywood.
By bevelling the clamping hole, to pass over the conical section of the pinion housing, the support material could be thickened if desired. If you imagine the plywood prototype as black plastic, or black Tufnol, it will all but disappear from view. I have some 10mm black Tufnol which doesn't smell. I presume the laminating resin only off-gasses when the material is freshly cut.
Note the complete lack of any extension outside the end of the fine focus knob. Many commercial and very expensive focusing motors, stick out sideways from the end of the focuser shaft. Some even add insult to injury by adding a stiff cable as well. Which is sticking out of a socket in the end of the already protruding motor! They really aren't thinking this through!
The lower curve on the plywood could be deepened and shaped for a sleeker look. Not to mention even better access to the brake screw and focus knobs for manual adjustment. The 29mm clamping hole must obviously be avoided but plastics and Tufnol are much stronger and stiffer materials, in smaller cross sections, than 9mm plywood.By bevelling the clamping hole, to pass over the conical section of the pinion housing, the support material could be thickened if desired. If you imagine the plywood prototype as black plastic, or black Tufnol, it will all but disappear from view. I have some 10mm black Tufnol which doesn't smell. I presume the laminating resin only off-gasses when the material is freshly cut.
Note the complete lack of any extension outside the end of the fine focus knob. Many commercial and very expensive focusing motors, stick out sideways from the end of the focuser shaft. Some even add insult to injury by adding a stiff cable as well. Which is sticking out of a socket in the end of the already protruding motor! They really aren't thinking this through!
Such ugly and potentially dangerous protuberances are totally unnecessary. As seen with my very simple set-up. The motor is slung completely out of the way under the focuser. Yet does not inhibit focuser movement. Nor deny access to the pinion brake thumbscrew.
It has now occurred to me that I could have a compression spring under the motor mount. To allow the user to make the toothed belt go slack for manual focusing. Simply by depressing the motor body upwards against the spring. Once the motor is released again the focuser returns to motor drive.
The reason I keep on about manual focusing is the very slow speed of the "Skywatcher" DC motor even at fastest setting on a brand new 9V battery. I see myself using manual adjustment for rapid, long travel adjustments and then relying on the motor for "hands free" fine focusing.
I think I might turn my own [half round cross section] pulley for an O-ring drive. The narrower O-ring would not hide the fine focus knob, like a wider belt. But should easily drive the focuser if the weak rubber band is anything to go by.The O-ring and pulley could easily be duplicated if need be. Lifting and [perhaps] lowering of heavy accessories is a whole different matter to winding an empty focuser drawtube along on a flat surface. Perhaps a toothed belt is the right way to go.
To avoid using Tufnol I have ordered a black, plastic, kitchen, cutting board, 1cm thick x 20 x 30cm. [0.4" x 8" x 12"] Enough [HDPE?] raw material to last me quite a while. It turned up the day after ordering and is superbly finished in semi-matt black with a diagonal grain. I also found an O-ring I had forgotten I had. About 60mm ØD x 4mm Ø. You couldn't ask for a better match for my needs.Perhaps 3mm would have had more stretch. I dn't want to put any side loads o the fine focus knob. Which I doubt they are really designed to cope with.
Click on any image for an enlargement.
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